Various aluminate phosphors have been put to practical use as phosphors for energy-saving fluorescent lamps. Examples of blue phosphors include (Ba,Sr)MgAl10O17:Eu (BAM:Eu), and examples of green phosphors include CeMgAl11O19:Tb and BaMgAl10O17:Eu,Mn.
In recent years, BAM:Eu, which exhibits high luminance under vacuum-ultraviolet excitation, has been used as a blue phosphor for PDPs.
However, when a light-emitting device using the blue phosphor BAM:Eu is driven for a long period of time, the luminance is degraded significantly. Hence, for use in a light-emitting device, particularly in a PDP, there is a strong demand for a phosphor that shows less luminance degradation even after long-time driving.
The mechanism of luminance degradation of the blue phosphor BAM:Eu has not been clarified sufficiently. The luminance degradation of the blue phosphor is likely to occur because of the entry of moisture and impurity gas as well as the heat treatment in the process of manufacturing the light-emitting device, and of the vacuum ultraviolet light irradiation during driving of the light-emitting device.
In order to prevent the luminance degradation, there have been proposed a method in which gadolinium is added to a phosphor (see JP 6 (1994)-29418 B, for example), a method in which a phosphor is coated with silicate of divalent metal such as alkaline earth metal (see JP 2000-34478 A, for example), and a method in which a phosphor is coated with an antimony oxide (see JP 10 (1998)-330746 A, for example).
However, light-emitting devices using the phosphors according to these conventional methods, in most cases, cannot suppress the luminance degradation of the phosphor during driving while maintaining high luminance.